The present invention relates to the manufacture of and/or subsequent processing of yarns and more particularly relates to a method and apparatus for reducing the moisture content of wet yarns to desired levels.
In the manufacture of wet-spun yarns or other yarns requiring aqueous washings or extracting before packaging, a drying process is utilized during which the moisture content of the wet yarns is reduced. Drying is often accomplished by supplying the wet yarn to a pair of spaced-apart drier rolls which spirally advance the yarn along the rolls while the yarns are progressively dried to a desired moisture level. Drier roll apparatus as disclosed in U.S Pat. No. 4,644,668 is advantageously used for this purpose.
In the manufacture and/or subsequent processing of some yarns, it is necessary to control the drying step so that the moisture content of the yarn (MOY) is within a certain range. An example of such a process is the manufacture of precursor yarns to make high modulus para-aramid yarns such as those disclosed in EPO Publication No. 247889, published Dec. 2, 1987. In the manufacture of such yarns, water-swollen precursor yarns are subject to heat treatment under tension to increase the modulus and tenacity of the finished yarn. The maximum benefits are obtained when the yarns are "never-dried", i.e., they are reduced in moisture content after spinning only to a certain relatively high MOY and then are subjected to further treatment. In certain dye-imbibing processes for aramid yarns, it is also desirable for precursor yarns to have a certain MOY before entering the dye bath. Moreover, in the drying of many types of yarns it is generally desirable for dried yarns to have a carefully controlled moisture content at packaging so that the package weight is an accurate measure of the yarn in a package.
Known equipment used for the drying of yarns is not well-suited for producing yarns with a controlled, high MOY. Typically, steam-heated drier rolls are designed to efficiently dry yarns to a very low moisture content when supplied with steam at a specified pressure. If the rolls are used for yarns desired to have a higher moisture content, the steam pressure must be decreased to reduce the extent of drying. However, in some equipment designed for substantially complete drying, the steam pressure often cannot be decreased to produce the desired reduced drying effect due to the build-up of condensate in the roll which can cause the roll to become inoperable.
Moreover, known equipment typically cannot respond to process changes which affect the degree of drying. For example, if ends are lost in a continuous filament yarn, the drying capacity of the drier roll apparatus can exceed the desired level of drying. Typically, the heat being supplied to the roll cannot be changed quickly and thus over-drying will result until the heat has been reduced as necessary. In steam-heated rolls, fluctuations in the supplied steam pressure can result in either temporary under-drying or over-drying of the yarn even when the steam supply is quickly re-adjusted.